The invention is directed to devices using high velocity liquid jets for cleaning perforated, slotted and wire-wrapped well liners which become plugged with foreign material.
In the well producing art, it is customary to complete wells, such as water, oil, gas, injection, geothermal, source, and the like, by inserting a metallic well liner adjacent a fluid producing formation. Openings in the well liner provide passage-ways for flow of fluids, such as oil or water and other formation fluids and material from the formation into the well for removal to the surface. However, the openings, which, for example, may be slots preformed on the surface or perforations opened in the well, will often become plugged with foreign material, such as products of corrosion, sediment deposits and other inorganic complexes. Analysis of the plugging material has shown it to be fine sand grains cemented together with oxides, sulfides and carbonates. Some asphaltenes and waxes are also present. Where water is produced, scale also seems to be a very tough plugging material. Removal and replacement of the liner is costly and is only a temporary solution as the liner will eventually again become plugged.
Jetted streams of liquid have been heretofore used to clean openings. The use of jets was first introduced in 1938 to directionally deliver acid to dissolve carbonate deposits. Relatively low velocities were used to deliver the jets. However, this delivery method did improve the results of acidizing. In about 1958 the development of tungsten carbide jets permitted including abrasive material in a liquid which improved the ability of a fluid jet to do useful work. The major use of abrasive jetting has been to cut notches in formations and to cut and perforate casing to assist in the initiation of hydraulically fracturing a formation. The abrasive jetting method requires a large diameter jet orifice. This large opening required an unreasonably large hydraulic power source in order to do effective work. The use of abrasives in the jet stream permitted effective work to be done with available hydraulic pumping equipment normally used for cementing oil wells. However, the inclusion of abrasive material in a jet stream was found to be an ineffective perforation cleaning method in that it enlarged the perforation which destroyed the perforation's sand screening capability.
More recently, Chevron Research Company disclosed a method and apparatus for directionally applying high pressure jets of fluid to well liners in a number of U.S. patents. These patents are U.S. Pat. Nos. 3,720,264, 3,811,499, 3,829,134, 3,850,241, and 4,088,191, which are herein incorporated by reference.
The assignee of the subject application is a licensee of the Chevron Systems and developed a cleaning operation and device pursuant to the Chevron disclosures. This system, which will hereinafter be referred to as the old design, employed a jet carrier of about 6 feet in length having 8 jet nozzles widely spaced along its length. The nozzles were threadably mounted on extensions which were in turn welded to the jet carrier. A fixed tri-blade pilot bit was affixed to the lower end of the jet carrier.
The jet carrier was attached to a tubing string that could be reciprocated and rotated within the well bore. As the carrier was moved and rotated adjacent the liner, the nozzles directed jet streams which contacted and cleaned the liner.
The distance between the well liner and the end of the jet nozzle, called the standoff distance, is critical in such a process. If this distance becomes too great, the power of the jet stream against the liner drops off markedly (decreasing proportional to the square of the distance). Conversely, if the standoff distance becomes too small, the power of the jet stream is reduced to a laminar flow along the well liner. Such a flow is also inadequate for proper cleaning. Thus, it has been found that the standoff distance should be between about 5 and 10 times the diameter of the jet nozzle orifice.
In the old design, the only means to regulate the standoff distance was to vary the diameter of the jet carrier itself. Thus, four different sizes of jet carriers were available for differently sized well liners. The Chevron patents disclose that liner adjustments can be made by turning the jet nozzles in and out of the welded extensions. However, in practice, this was found not to be workable because of leakage if the nozzles are not tight in the extensions.
A function of the pilot bit was to provide mechanical centralization of the jet tool during running of the tool in the well. Thus, the blades of the pilot bit were selected to be slightly less in diameter than the inside diameter of the liner.
This old design, although a significant advance in the art, developed a number of problems. First, the sprial track created by the streams of fluid from the nozzles as the tool was lifted and rotated, were widely spaced from each other. In order to ensure coverage of each point on the liner, the vertical and rotational speed had to be decreased to levels which resulted in fluid coverage of other points on the liner of three or more times. Conversely, if the rotational and vertical speeds were increased to reduce the coverage of areas to no more than twice, significant areas on the liner remained uncleaned. This caused substantial inefficiencies in the cleaning process.
Secondly, the wide spacing of the nozzles required a long jet carrier which was prone to jamming in well bores which deviated significantly from vertical.
Thirdly, since the extensions were permanently welded to the carrier, a different size jet carrier was required for each size of well liner. To retain such an inventory of carriers was expensive and produced logistic complications in the transportation of the various sized carriers to on-site cleaning operations. Accordingly, only four sizes of jet carriers were available for the various sizes of well liners. As a result, control over the standoff distance was inexact and limited, and the required standoff distances of about 5 to 10 times the diameter of the nozzle orifice were often being violated. In a few cases, the distance was exceeded by 10 to 15 diameters.
A fourth problem was the possibility of weld failure at the extension/carrier interface.
A fifth problem was the inability of the fixed pilot bit to ensure concentric rotation of the assembly. At times in "tight hole" situations, the assembly was prone to jam or produce back-torque due to liner contact. If the fixed pilot bit were made smaller to reduce jamming, the standoff distance could vary resulting in nozzle peening or closing of the jet orifices.
Due to the multitude of problems which developed with the old design, a strong need existed for a device and method of cleaning well liners in which every perforation on any given liner could be cleaned more effectively and efficiently in a more practical, economical operation.